This application is based on German Application DE 199 43 057.8, filed Sep. 9, 1999, which disclosure is incorporated herein by reference.
The invention relates to a pyrogenically prepared silicon dioxide doped with silver or silver oxide by means of an aerosol and having bactericidal properties, and to a process for its preparation and its use. The invention relates also to use of this doped silicon dioxide in dispersions or as a filler in rubber and silicone rubber.
It is known to dope pyrogenically prepared silica in one step in a flame using a special process. (DE 96 50 500 A1). That process is a combination of high-temperature flame hydrolysis with pyrolysis. That doping process is to be distinguished from the older, so-called xe2x80x9cco-fumed processxe2x80x9d, in which the gaseous starting materials (for example SiCl4 gas and AlCl3 gas) are pre-mixed and burnt together in a flame reactor.
In the doping process, an aerosol is fed into a flame in which a pyrogenic oxide is produced by flame hydrolysis, the aerosol containing a salt of the compound to be doped.
It has now been found that if silver salts dissolved in water are used as the starting material for the aerosol, a doped pyrogenic silica having bactericidal properties is obtained as the product.
The invention provides a pyrogenically prepared silica doped with silver or silver oxide by means of an aerosol. The silica is characterized in that the base component is a silica prepared pyrogenically by flame oxidation or, preferably, by flame hydrolysis. The silica is doped with a doping component of from 0.0001 wt. % up to 20 wt. %, the doping amount preferably being in the range from 1 to 10,000 ppm, and the doping component is a salt or a salt mixture of a silver compound or metallic silver or mixtures thereof. The BET surface area of the doped oxide is from 1 to 600 m2/g, preferably in the range from 40 to 400 m2/g.
The invention further provides a process for the preparation of the pyrogenically prepared silica doped with silver or silver oxide by means of an aerosol, which process is characterized in that an aerosol is fed into a flame such as is used for the pyrogenic preparation of silica by flame oxidation or, preferably, by flame hydrolysis, the aerosol is mixed homogeneously with the gas mixture used for the flame oxidation or the flame hydrolysis before the reaction, then the aerosol/gas mixture is allowed to react in the flame and the resulting pyrogenically prepared silicas doped with silver or silver oxide are separated from the gas stream in a known manner. An aqueous solution containing salts or salt mixtures of silver or the metal itself in dissolved or suspended form or mixtures thereof is used for the preparation of the aerosol, The aerosol is produced by atomization by means of a two-component nozzle or by a different method of aerosol production.
The following may be used as salts: Ag(NO3), Ag2(SO4), Ag2O, or the salts may be complexed with complexing agents or ammonia.
The process of flame hydrolysis for the preparation of pyrogenic oxides is known from Ullmanns Enzyklopxc3xa4die der techn. Chemie 4th edition, Vol. 21, p. 464.